Steering stability and performance of a vehicle are largely characterized by the vehicle's understeer and oversteer behavior. The vehicle is in an understeer condition if the vehicle yaw is less than the operator steering input, where turning the steering wheel more does not correct the understeer condition because the wheels are saturated. The vehicle is in an oversteer condition if the vehicle yaw is greater than the operator steering input. Surfaces such as wet or uneven pavement, ice, snow or gravel also present vehicle stability and handling challenges to the operator. Similarly, in a panic or emergency situation, such as during obstacle avoidance, an operator may react by applying too much steering or failing to counter-steer to bring the vehicle back to its intended path. In any of these cases, the actual vehicle steering path deviates from the intended steering path.
Vehicle stability controls have progressed from first generation systems based upon braking and traction control (braking and powertrain torque management) technologies to more recent systems including independent and coordinated controls of brake, powertrain, steering and suspension damping sub-systems. Typically, distributed control modules are employed to directly interface with respective actuators to effect the desired sub-system controls. Coordination and authority of such sub-system control may be handled by way of a supervisory control.
Braking and traction control sub-systems can effect understeer and oversteer stability enhancements. Such sub-systems rely on wheel speed, steering angle, vehicle speed, yaw rate and other considerations to reduce engine torque and apply vehicle braking to maintain the vehicle travel along the intended path.
Active front steering sub-systems can effect understeer and oversteer stability enhancements. Such sub-systems employ a steering actuator system that relies upon an operator intended steering input from a hand wheel sensor, vehicle speed, vehicle yaw rate and other considerations, and provides a correction to the operator steering input to cause the vehicle to more closely follow the vehicle operator's intended steering path to increase vehicle stability and improve vehicle handling.
Semi-active suspension systems are also incorporated into some modern vehicles and are generally characterized by dampers that are controlled to change the suspension characteristics of the vehicle based on road conditions, vehicle speed, yaw rate and other considerations. Variable fluid-based dampers are known having discrete damping states and continuously variable damping states which affect both jounce and rebound response of the suspension system. Variability in damping may be attained by variable orifice devices or controlled viscosity fluids (e.g., magnetorheological (MR) or electrorheological (ER)) within the damping device. Variable dampers are used predominantly to achieve low speed ride comfort and high speed handling enhancement (ride and handling). However, variable damping techniques are known to enhance vehicle stability in certain understeer and oversteer situations and may be implemented as part of an overall vehicle stability control.
Whether implemented independently, overlapped or integrated, braking, traction, steering and suspension-based stability enhancement sub-systems all rely upon certain common vehicle level parameters. And, vehicle dynamics information, including vehicle steering response (e.g., understeer, oversteer or neutral steer), vehicle turning direction (e.g., left or right) or combinations thereof, defining vehicle dynamics states is commonly employed across stability enhancing sub-systems. Effective stability control systems, therefore, benefit from integrity and flexibility of use of such vehicle dynamics information.
Systematic reuse of control components—both within a vehicle's control architecture and across vehicle platforms and applications—promotes low-cost, quick-to-market and widely available vehicle systems. Significant benefits result directly from the application development cost, time, validation, maintainability and flexibility advantages afforded by such common control assets. Therefore, it is desirable that a vehicle stability enhancement system be characterized by a high degree of control component availability and access to enable and promote reuse, maintainability, common validation and development, cost and time savings and multi-platform utilization.